Selecting all faces on a plane on imported mesh
Option 1: press shift+g and select normal. This will select all faces with the same orientation.

Option 2: Press some number on the numpad to snap your view to a specific axis. Snap you view so that all of the faces are seemingly 2 dimensional, and you are looking at the from the side. Then you can press z to change to wire frame mode. Next, press b (box select) and drag a box across all the vertices you want.